add php 8.1 to build matrix (#19)

Author: iamcal

.github/workflows/php.yml

@@ -45,6 +45,9 @@ jobs:
           operating-system: 'ubuntu-20.04'
           phpunit-versions: '7.5.20'
           coverage: true
+        - php-versions: '8.1'
+          operating-system: 'ubuntu-20.04'
+          phpunit-versions: '9.5.4'
 
     steps:
     - name: Setup PHP